Radio Frequency Fundamentals

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Nov 16, 2013 (3 years and 9 months ago)

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Radio Frequency Fundamentals

Chapter 02

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CWNA®: Certified Wireless Network
Administrator Official, Study Guide, David
D. Coleman & David A. Westcott, Sybex

1

What Will I Learn

We will concentrate on the basics of
radio frequency signals. To
properly
design and administer a WLAN network, it is essential to have a
thorough
understanding

of
the following
principles of RF properties

and
RF

behaviors
:



Electromagnetic
waves and how they are
generated



The
relationship between wavelength, frequency, and the speed of
light



Signal
strength and the various ways in which a signal can either attenuate
or
amplify



The
importance of the relationship between two or more
signals



How
a signal moves by bending, bouncing, or absorbing in some
manner


Customized by: Brierley

CWNA®: Certified Wireless Network
Administrator Official, Study Guide, David
D. Coleman & David A. Westcott, Sybex

2

What Will I Learn


When troubleshooting an
Ethernet

network, the best place to start is
always
at layer 1,
the
Physical layer
.


WLAN

troubleshooting should also begin at the
Physical layer
.


Learning
the RF fundamentals that exist at layer 1 is an essential step in
proper wireless network
administration
.

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CWNA®: Certified Wireless Network
Administrator Official, Study Guide, David
D. Coleman & David A. Westcott, Sybex

3

Key Terms


absorption


active gain


alternating current

(
AC)


amplification


amplitude


attenuation


delay spread


diffraction


downfade


electromagnetic (EM)

spectrum


free space path loss

(
FSPL)


frequency


gain


hertz (Hz)


intersymbol


interference
(ISI)


link budget


loss


microwave


multipath


noise floor



nulling


oscillation


oscilloscope


passive gain


phase


propagation


propagation behavior


radio frequency (RF)


Rayleigh fading


received amplitude


reflection


refraction


RF shadow


scattering


site survey


spectrum analyzer


transmit amplitude


upfade


wavelength

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4

Key Topics


3.1
. RF Components


3.2
. Units of Power and Comparison


3.3
. RF Mathematics


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5

Discussion Topics


Definition of
radio frequency
signal


Radio frequency
characteristics


Wavelength


Frequency


Amplitude


Phase


Radio frequency
behaviors


Wave
propagation


Absorption


Reflection


Scattering


Refraction


Diffraction


Loss (attenuation
)


Free space path
loss


Multipath


Gain (amplification)

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6

Electromagnetic Spectrum

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7

What Is a Radio Frequency (RF) Signal?


The electromagnetic (EM) spectrum
, which is usually simply referred to as
spectrum, is the range of all possible electromagnetic radiation.



This radiation exists as self
-
propagating electromagnetic waves that can
move through matter or space. Examples of electromagnetic waves
include gamma rays, X
-
rays, visible light, and radio waves.



Radio waves are electromagnetic waves occurring on the
radio frequency
portion of the electromagnetic spectrum
.



An RF signal starts out as an electrical alternating current (AC) signal that
is originally generated by a
transmitter
.




This AC signal is sent through a copper conductor (like a coaxial cable) and
radiated out of an antenna element in the form of an electromagnetic
wave.


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D. Coleman & David A. Westcott, Sybex

8

What Is a Radio Frequency (RF) Signal?


This
electromagnetic wave
is also the
wireless signal. Changes of electron flow in
an antenna, otherwise known as current, produce changes in the electromagnetic
fields around the antenna
.



The shape and form of the AC signal

defined as the waveform

is what is known
as a
sine wave
.



The fluctuation of voltage in an AC current is known as
cycling
, or
oscillation
.



RF electromagnetic signals travel using a variety or combination of
movement
behaviors
.



These movement behaviors are referred to as
propagation behaviors
. We will
discuss some of these propagation behaviors, including absorption, refection,
scattering, refraction, diffraction,
amplification
, and attenuation.


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D. Coleman & David A. Westcott, Sybex

9

Radio Frequency Characteristics

These characteristics,
defined
by the laws of
physics, exist in every RF signal:


Wavelength


Frequency


Amplitude


Phase


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Sine Wave

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Wavelength

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Wavelength

RF
signal is an alternating current (AC) that continuously changes between a
positive and negative voltage.

An
oscillation
, or cycle, of this alternating current is defined as a single change
from up to down to up, or as a change from positive to negative to positive.

A
wavelength

is the
distance between the two successive crests
(peaks) or
two successive troughs (valleys) of a wave pattern.



In simpler words, a wavelength is the distance that a single cycle of an RF
signal actually travels.




Wavelength = λ measured in meters(m)


Frequency= f measured in Hz


Light =C = 300,000,000 m/sec (a constant


the speed of light) = 300M/sec





λ = c/f


f = λ/c


c = 300 M/sec





1 Hz = 1 cycle/sec



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13

Wavelength



As RF signals travel through space and matter, they lose signal strength (
attenuate
).



The
higher frequency
signal has just become too weak that it is below the receive sensitivity
threshold of the receiving radio, whereas the lower frequency signal is still above the receivers
sensitivity threshold
.


An electromagnetic signal with
a larger wavelength
will maintain an amplitude level above the
sensitivity of a receiver radio over greater distances
.


The higher frequencies will attenuate faster through space.


This is important for a wireless engineer to know for two reasons
.




First, the coverage distance is dependent on the attenuation through the air (referred to as
free space path loss (
FSPL
).



Second, the higher the frequency, the less the signal will penetrate through obstructions.
For example, a 2.4 GHz signal will pass through walls, windows, and doors with greater
strength than a 5 GHz signal.


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14

750 KHz & 252 GHz wavelengths

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254 GHz & 5.776 GHz wavelength

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16

Frequency


Frequency is the number of times a
specified
event
occurs within a
specified
time interval
.




A standard measurement of frequency is hertz (Hz),
which was named after the German
physicist
Heinrich
Rudolf Hertz
.



λ = c/f and f = c/λ
.
A simplified explanation is that
the
higher the frequency
of an RF signal
shorter the
wavelength
will be of that signal
.



The longer the wavelength of an RF signal, the lower
the frequency will be of that signal.


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17

Frequency

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18

Amplitude

Characterized simply as the signal’s strength, or power.


A variety of factors can cause an RF signal to lose amplitude, otherwise known
as
attenuation
.


Different types of RF technologies require varying degrees of transmit
amplitude
.



AM radio stations may transmit narrow band signals with as much power as
50,000 watts.


The radio cards in most indoor 802.11 access points have a transmit power
range between 1
mW

and 100
mW
.


You
will learn later that Wi
-
Fi radio cards can receive signals with amplitudes as
low as billionths of a
milliWatt
.

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19

Phase

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Phase

What is important to understand is the effect that phase has on amplitude when radio cards
receive multiple signals.


Signals that have
0 (zero) degree phase separation
actually combine their amplitude, which
results in a received signal of much greater signal strength, or
twice the amplitude
.


If two RF signals
are 180 degrees out of phase
(the peak of one signal is in exact alignment
with the trough of the second signal), they cancel each other out and the effective received
signal strength is
null
.



Phase separation has a cumulative effect
.


Depending on the amount of phase separation of two signals, the received signal strength may
be either increased or diminished.


The phase difference between two signals is very important to understanding the effects of an
RF phenomenon

known as
multipath
.


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21

Radio Frequency Behaviors

As a WLAN engineer, it is important to understand RF propagation behaviors for
making sure that access points are deployed in the proper location, for making sure
the proper type of antenna is chosen, and for monitoring the health of the wireless
network.



Wave Propagation


What happens as the wave moves away from an antenna.


multipath,


attenuation,


gain



RF propagation behaviors include: (how the wave moves)


absorption,


refection,


scattering,


refraction,


diffraction,


free space path loss,


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22

Propagation Analogy

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23

Absorption

The most common
RF behavior
is absorption.


If a signal does not:



bounce off an object,



move around an object, or



pass through an object,



then 100 percent absorption has
occurred


Most materials will absorb some amount of an
RF signal to varying degrees.

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24

Reflection

One of the most important RF propagation behaviors to be aware of is refection.



When a wave hits a smooth object that is larger than the wave itself, depending on the media,
the
wave
may bounce in another direction. This behavior is categorized as refection.



There are two major types of
reflections
:


sky wave refection and


microwave refection.



Sky wave refection can occur in frequencies below 1 GHz, where the signal has a very large
wavelength.



The signal bounces off the surface of the charged particles of the ionosphere in the earth’s
atmosphere.



Microwave signals, however, exist between 1 GHz and 300 GHz. Because they are higher
-
frequency signals, they have much
smaller wavelengths
, thus the
term microwave
.

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25

Reflection

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26

Scattering

Did you know that the color of the sky is blue

because
the wavelength of light is smaller than the
molecules of the atmosphere?



This blue sky phenomenon is known as Rayleigh
scattering.


The shorter blue wavelength light is absorbed by the
gases in the atmosphere and radiated in all directions.

This is another example of an RF propagation behavior
called scattering, sometimes called scatter
.


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27

Scattering

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28

Refraction

In addition to RF signals being absorbed or bounced (via refection or
scattering), if certain conditions exist, an RF signal can actually be bent
in a behavior known as
refraction
.


A straightforward definition of refraction is the bending of an RF signal
as it passes through a medium with a different density, thus causing
the direction of the wave to change.



RF refraction most commonly occurs as a result of atmospheric
conditions.



When you are dealing with long
-
distance outdoor bridge links, an
instance of refractivity change that might be a concern is what is
known as the k
-
factor.


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29

Refraction


A k
-
factor of 1 means there is no bending.


A k
-
factor of less than 1, such as 2/3, represents the signal bending away from
the earth.


A k
-
factor of more than 1 represents bending toward the earth.


Normal atmospheric conditions have a k
-
factor of 4/3, which is bending
slightly toward the curvature of the earth.


The three most common causes of refraction are:



water vapor,



changes in air temperature, and



changes in air pressure.


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30

Refraction

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31

Diffraction

Not to be confused with refraction.



Diffraction is the bending of an RF signal around an object (whereas
refraction, as you recall, is the bending of a signal as it passes through a

medium).



Diffraction is the bending and the spreading of an RF signal when it
encounters an obstruction.



The conditions that must be met for diffraction to occur depend entirely on
the shape, size, and material of the obstructing object as well as the exact
characteristics of the RF signal, such as:


polarization,


phase, and


amplitude.


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32

Diffraction Analogy

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Loss (
Attenuation
)


Loss, also known as attenuation, is best described as the
decrease of
amplitude, or signal strength.





A signal may lose strength while on a wire or in the air.





On the wired portion of the communications (RF cable), the AC electrical
signal will lose strength because of the electrical impedance of coaxial cabling
and other components such as connectors.





RF engineer may add a hardware attenuator device on the wired side of an
RF system to introduce attenuation to remain compliant with power
regulations or for capacity design purposes.





Water is a major source of absorption as well as dense materials such as
cinder blocks, all of which lead to attenuation.


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34

POTENTIAL
LAB

Visual Demonstration of absorption In this exercise, you will use a program called EMANIM, a freeware
program found on the CD that comes with this book, to view the attenuation effect of materials due to
absorption
.


1. Insert the CD included with this book into your computer and install the EMANIM program by
double
-
clicking emanim_setup.exe.

2. From the main EMANIM menu, click Phenomenon.

3. Click
Sybex

CWNA Study Guide.

4. Click Exercise 2.2.

5. When a radio wave crosses matter, the matter absorbs part of the wave.



As a result, the amplitude of the wave decreases.



The extinction
coeffcient

determines how much of the wave is absorbed by unit length of material.



Vary the length of the material and the extinction
coeffcient

for Wave 1 to see how it affects the
absorption.


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35

Free Space Path Loss

Free space path loss (
FSPL
) is the loss of signal strength caused by the natural
broadening of the waves, often referred to as beam divergence.



RF signal energy spreads over larger areas as the signal travels farther away
from an antenna, and as a result, the strength of the signal attenuates.



Luckily, this loss in signal strength is logarithmic and not linear; thus the
amplitude does not decrease as much in a second segment of equal length as
it decreases in the
first
segment.



A 2.4 GHz signal will change in power by about 80 dB after 100 meters but will
lessen only another 6 dB in the next 100 meters.


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36

Multipath

Multipath is a
propagation phenomenon

that results in two or more paths of a signal
arriving
at
a receiving antenna at the same time or within nanoseconds of each other.



Due of the natural broadening of the waves, the propagation behaviors of refection,
scattering, diffraction, and refraction will occur differently in dissimilar environments.




A signal may reflect off an object or scatter, refract, or diffract.



These propagation behaviors can all result in
multiple paths

of the same signal.



It usually takes a little bit longer for reflected signals to arrive at the receiving antenna

because they must travel a longer distance than the principal signal.



The time differential between these signals can be measured in billionths of a second
(nanoseconds).



The time differential between these multiple paths is known as the
delay spread

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Multipath

Downfade


This is
decreased signal strength
. When the multiple RF signal paths arrive

at the receiver at the same time and are out of phase with the primary wave, the result is a

decrease in
signal strength (
amplitude
). Phase differences of between 121 and 179 degrees

will cause
downfade
.



Upfade


This
is
increased signal strength
. When the multiple RF signal paths arrive at the

receiver at the same time and are in phase or partially out of phase with the primary wave,

the result is an
increase in
signal strength (
amplitude
). Smaller phase differences of between

0 and 120 degrees will cause
upfade
. Please understand, however, that the final received sig
-

nal

can never be stronger than the original transmitted signal because of free space path loss.



Nulling

This is
signal cancellation
. When the multiple RF signal paths arrive at the receiver

at the same time and are
180 degrees out of phase

with the primary wave, the result will be

nulling
. Nulling is the complete cancellation of the RF signal
.

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Multipath

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Multipath



The time between the primary signal and the reflected signals is known as the
delay spread
.



The
delay spread time differential
can cause bits to overlap with each other,
and

the end result is corrupted data, this interference is often known as
intersymbol

interference (
ISI
).



The good news is that the receiving station will detect the errors through an
802.11
-
defned cyclic redundancy check (CRC) because the checksum will not
calculate accurately.



Even so multipath can have a very negative impact on the performance or
throughput in your WLAN due to high incidents of rejected data
.

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Data Corruption ISI

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Gain: (Amplification
)

The two types of gain are known as:


active gain and


passive
gain


A
signal’s amplitude can be boosted by the use of external devices.



Active gain

is usually caused by the use of an
amplifier
on the wire that connects the

transceiver to the antenna
.



The
amplifer

is usually
bidirectional
, meaning that it increases the AC voltage both
inbound and
outbound
. Active gain devices require the use of an external power source.



Passive:



Passive gain is accomplished by focusing the RF signal with the use of an antenna.



Antennas are passive devices that do not require an external power source.



Instead, the internal workings of an
antenna focus the signal
more powerfully in one direction than
another.


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42

Measure
the
Amplitude
of a
Signal

Two very different tools can be used to measure the
amplitude of a signal at a given
point
.


a
spectrum analyzer

-

A
frequency

domain tool
, can
be used to measure amplitude in a finite frequency
spectrum
.


an
oscilloscope

-

A
time

domain tool
, can be used
to measure how a signal’s amplitude changes over
time.


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RF Signal Measurement Tools

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44

What Did I Learn

We concentrated on the basics of
radio frequency signals. To
properly
design
and administer a WLAN network, it is essential to have a
thorough
understanding

of
the following
principles of RF properties

and
RF

behaviors
:



Electromagnetic
waves and how they are
generated



The
relationship between wavelength, frequency, and the speed of
light



Signal
strength and the various ways in which a signal can either attenuate
or
amplify



The
importance of the relationship between two or more
signals



How
a signal moves by bending, bouncing, or absorbing in some
manner


Customized by: Brierley

CWNA®: Certified Wireless Network
Administrator Official, Study Guide, David
D. Coleman & David A. Westcott, Sybex

45

What Did I Learn


When troubleshooting an
Ethernet

network, the best place to start
is
always
at layer 1,
the
Physical layer
.


WLAN

troubleshooting should also begin at the
Physical layer
.


Learning
the RF fundamentals that exist at layer 1 is an essential step in
proper wireless network
administration
.

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Administrator Official, Study Guide, David
D. Coleman & David A. Westcott, Sybex

46

End

Ch

02
-

Radio
Frequency
Fundamentals

Next

Ch

03
-

Radio Frequency Components,
Measurements, & Mathematics

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